1. Field of the Invention
The present invention relates to an optical element used for extreme ultraviolet radiation, an exposure apparatus using this, and a device manufacturing method.
2. Description of Related Art
In conjunction with the miniaturization of semiconductor integrated circuits in recent years, exposure technology has been developed which, instead of conventional ultraviolet radiation, uses extreme ultraviolet radiation composed of wavelengths (11-14 nm) that are shorter than this conventional radiation in order to enhance optical system resolution that is attained according to the diffraction limits of light. By this means, it is anticipated that exposure light with a pattern size of approximately 5-70 nm will be feasible, and as the refractive index of materials of this region approaches 1, transmission-refractive type optical elements cannot be used as heretofore, and reflective optical elements are used. For reasons also of transmissivity assurance and the like, the mask used in the exposure apparatus is that of an ordinary reflective optical element. In this regard, in order to attain a high reflectance in each optical element, it is common to alternately laminate atop a substrate a material with a high refractive index and a material with a low refractive index in the employed wavelength region (see, e.g., Japanese Unexamined Patent Application, First Publication No. 2003-14893).